I've finally bothered to do the math for this.. and this is what I came up with...
Damage = (Caster's Attack Power * 2 - Target's Magic Defense) * Spell Multiplier
Damage = Damage + (Damage * rand(100,150)/100)
This monster spell is pretty powerful, but is totally underused. JCE's hack takes full advantage of this (although I doubt he knew where the #s came from, but it works perfectly for his modified Behemoth).

Note that this spell is affected by Magic Evade, which will affect the Spell Multiplier.